This invention relates to a hologram recording and reproduction apparatus, and more particularly to a hologram recording medium for preventing useless sensitization thereof by unnecessary light and a method of producing a hologram recording medium of the type described.
In recent years, a hologram recording and reproduction apparatus which makes use of a hologram technology to record and reproduce a large amount of data has been proposed and is disclosed, for example, in “Holographic data storage”, IBM J. RES DEVELOP, Vol. 44, No. 3, May 2000. The hologram recording and reproduction apparatus has such a general configuration as shown in FIG. 9.
Referring to FIG. 9, a coherent laser beam emitted from a laser light source 41 enters a beam splitter 43 through a shutter 42 and is split into signal light 100 and reference light 200 by the beam splitter 43. The signal light 100 enters a spatial light modulator 45 through a mirror 44 and is intensity modulated by the spatial light modulator 45 on which a data page is displayed. The modulated signal light 100 is condensed into a hologram recording medium 30 by a lens 46. Meanwhile, the reference light 200 enters a lens 51 through a mirror 50 and is illuminated on the hologram recording medium 30 through the lens 51. Consequently, the signal light 100 and the reference light 200 interfere with each other in the hologram recording medium 30, and interference fringes formed from the signal light 100 and the reference light 200 are recorded as a fine density pattern in the hologram recording medium 30.
In order to reproduce data recorded in the hologram recording medium 30, illumination reference light same as the reference light 200 is illuminated on the hologram recording medium 30, whereupon the data are reproduced as diffraction light corresponding to the interference fringes recorded in the hologram recording medium 30. The diffraction light is focused on an image pickup apparatus 49 such as a CCD pickup apparatus or a CMOS pickup apparatus by a lens (inverse Fourier lens) 48. The image pickup apparatus 49 photoelectrically converts the received diffraction light and analyzes a resulting received light signal to reproduce the same as image data.
Such a hologram recording and reproduction system as just described uses a technique called multiplex recording to achieve a high recording density. Different from conventional recording techniques, the multiplex recording records a large number of pages at a place. As representative ones of such multiplex recording methods, angle multiplex recording, shift multiplex recording and phase code multiplex recording are available. Further, various other multiplex recording methods such as speckle multiplex recording are known.
Incidentally, whichever multiplex recording method such a hologram recording and reproduction system as described above uses, it has a problem that, when interference fringes produced by the signal light 100 and the reference light 200 are recorded into the hologram recording medium 30, a portion of the hologram recording medium 30 other than a region to be recorded originally is exposed to unnecessary light. This is described in detail with reference to the drawings.
FIG. 10 is a sectional view illustrating unnecessary light generated from the signal light 100 when interference fringes (data) are to be recorded into a hologram recording medium. The hologram recording medium 30 is formed from a pair of transparent cover glass plates 11 and recording material 12 such as photopolymer sandwiched between the cover glass plates 11. When the signal light 100 enters the hologram recording medium 30 through one of the cover glass plates 11, it passes through a recording region A of the recording material 12 and goes out from the cover glass plate 11 on the opposite side. Thereupon, the signal light 100 interferes with reference light not shown at a portion of the recording region A, whereupon interference fringes are recorded into the portion of the recording region A of the recording material 12.
However, when the signal light 100 enters the hologram recording medium 30, unnecessary light 400 is actually generated which includes scattered light generated by scattering of part of the signal light 100 by the recording material 12 and reflected light by surface reflection at an interfacial boundary between each of the cover glass plates 11 and the recording material 12 or the air. The unnecessary light 400 propagates between the paired cover glass plates 11 disposed in an opposing relationship to each other to a location around the recording region A and sensitizes the area of the recording material 12 other than the recording region A.
The unnecessary light 400 thus sensitizes a portion other than the portion (recording region A) which originally is to be sensitized, and gives rise to deterioration of a characteristic of the recording material 12 and further has a bad influence on the recording capacity (decreases the recording capacity by an amount of the sensitized portion). It is to be noted that a similar phenomenon is caused also by the reference light 200. For example, where the angle multiplex method is adopted wherein the angle of the reference light 200 is changed to record or reproduce a large number of independent data pages at one place, if the reference light 200 enters at such an angle as indicated by an arrow mark in FIG. 11, unnecessary light 400 is generated likewise by scattering by the material and the surface reflection and sensitizes a portion of the recording material 12 other than the recording region A. Naturally, unnecessary light other than those indicated in FIGS. 10 and 11 is generated.